Electrochemical Lithium Intercalation-Assisted Exfoliation of TMDs

Abstract

Transition metal dichalcogenides (TMDs) have attracted lots of attention due to the fancy properties of two-dimensional (2D) TMDs, such as unique band gaps, excellent electronic conductivity, and high catalytic activity. These properties endow 2D TMDs with great applications in electronics, energy storage and conversion, and catalysis. Thus, the preparation of 2D TMDs is demanded. The classical strategies contain bottom-up and top-down methods. The typical bottom-up method includes chemical vapor deposition (CVD) and wet chemical synthesis. The top-down approach contains scotch tape-based micromechanical exfoliation, liquid exfoliation, electrochemical exfoliation, chemical intercalation-assisted exfoliation, and electrochemical intercalation-assisted exfoliation. Among them, electrochemical lithium intercalation-assisted exfoliation is a controllable, safe, time-saving, and large-scale method that can be applied to many TMDs. Electrochemical lithium intercalation-assisted exfoliation comprises two steps: electrochemical lithium intercalation and the following exfoliation. However, too much attention was focused on the exfoliated TMDs instead of electrochemical lithium intercalation. Thus, a profound understanding of phase transformation during electrochemical lithium is lacking. Here, we focused on the overall process of electrochemical lithium intercalation-assisted exfoliation, including phase transformation during lithiation and exfoliated TMDs.

In-situ and ex-situ technologies, such as X-ray diffraction(XRD) and Raman, were applied to monitor the phase transformation during electrochemical lithium intercalation. Raman, X-ray photoelectron spectroscopy (XPS), and (scanning) transmission electron microscope ((S)TEM) were used to determine the structure of single/few-layer TMDs. TiS₂, WS₂ and ZrS₂ undergo a solid-solution reaction (single-phase reaction) during electrochemical lithium intercalation. MoS₂ and NbS₂ involve a double-phase reaction during electrochemical lithium intercalation, namely, 2H MoS₂ changes to 1T LiMoS₂ and 2H NbS₂ transfers to 3R LiNbS₂. 1T TaS₂ and 1T VS₂ undergo multi-phase reaction before reaching the 2H LiTaS₂ and 2H LiVS₂. The phase transition behavior varies with TMDs. Theoretical calculations demonstrate that lithium intercalation expands the interlayer spacing of MS₂ (except for WS₂), MS₂ tends to transfer to a new phase with a smaller M-S bond length, and the final product after lithium intercalation is energetically favorable. All these TMDs can be exfoliated after lithium intercalation. The product of LiZrS₂ exfoliation is amorphous, the products of LiTiS₂ exfoliation, LiVS₂ exfoliation, and LiTaS₂ exfoliation are 1T phase nanosheets, the product of Li_2.8WS₂ exfoliation is 2H and 1T’ phase WS₂ nanosheets, the product of Li_1.4MoS₂ exfoliation contains 2H phase and 1T’ phase MoS₂ nanosheets, and the product of Li_1.4NbS₂ exfoliation is H phase NbS₂ nanosheets.

In this work, we investigated the overall process of electrochemical lithium intercalation-assisted exfoliation of TMDs for the first time. We developed in-situ technologies to observe the structure evolution during electrochemical lithium intercalation, then we revealed the phase transformation behavior and mechanism during electrochemical lithium intercalation. Third, we exfoliated lithium-intercalated TMDs and obtained single/few-layer TMDs with various phases. Finally, we determined the phase of these single/few-layer TMDs. This research can enhance our understanding of electrochemical lithium intercalation-assisted exfoliation and provide a general strategy for preparing single/few-layer TMDs.

Date of Award	21 Jul 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhiyuan ZENG (Supervisor)